The effect of phosphorus in waste water on the lakes and streams into which such water flows is so serious that its removal is recognized to be beneficial, if not imperative. Attention is invited to U.S. House of Representatives Report 91-1004, pages 1 to 9.
When certain forms of phosphorus are present in lakes, rivers and streams, the resultant fertilization of the waters promotes the growth of algae and other undesirable vegetation. These are unsightly, produce obnoxious odors on decaying, prevent recreational use of the waters and may be toxic to aquatic, domestic and wild animals.
It is a paradox that conventional sewage treatment plants may actually increase the immediate availability of harmful phosphorus in natural bodies of water. Those plants which involve biological processes are examples, e.g., activated sludge plants, trickling filters and the like.
The activated sludge processes include diffusing air through influent waste water to develop suspended colonies of biological growths or organisms which thrive in the presence of oxygen. A biologically active sludge is thus formed. When the latter is contacted with incoming waste water in a reactor, the organisms and organic components in the water engage in biochemical reactions such as for instance bioprecipitation, adsorption, absorption and aqueous oxidation. The incoming waste water and semi-purified water which contains active microorganisms are introduced into and removed from the reactor on a continuous or periodic basis in a generally balanced relationship.
Following the reactor the semi-purified waste water which contains significant quantities of suspended materials is subjected to a step in the process wherein the mixture is clarified and the relatively clear liquid separated from that portion containing the greater quantity of suspended solids. This clarification may be accomplished in a number of ways including sedimentation, flotation, filtration, screening or centrifuging. The resultant clear fraction is in some instances sufficiently pure for reuse or disposal in the receiving water course. In some instances, other and additional treatment procedures such as further removal of dissolved or suspended materials are practiced prior to reuse or disposal.
The concentrated fraction which contains most of the suspended solids may either be subjected to further or other treatments, disposal, recycle to the reactor, or, more typically, a combination thereof. In the latter instance a given quantity of sludge contacts the same water a plurality of times before the water and sludge are finally removed from the systems. By recycling in this manner, up to 90% or more of the organic contaminants of the influent waste water can be eliminated.
Besides strictly organic waste, the influent of a sewage treatment plant very commonly contains significant quantities of mineral elements, including both insoluble and soluble forms of phosphorus, the latter being generally recognized as the form which is most readily used by algae and the like. While the microorganisms used in biological sewage treatment plants metabolically utilize some phosphorus, the microorganisms reportedly convert enough organically bound phosphorus to soluble form so that-- as compared to the influent-- the effluent may contain more of the soluble phosphorus so readily assimilated by the undesired vegetation. See U.S. Pat. No. 3,236,766. Moreover, it is recognized elsewhere that conventional biological sewage treatment plants are at best ill-equipped to cope successfully with phosphorus. See U.S. Pat. Nos. 3,423,309 and 3,390,077.
As a consequence of the serious problems resulting from phosphorus in waste waters, and the failure of conventional treatment facilities to remove significant quantities thereof, considerable effort has been and will continue to be devoted toward that end. Many methods are under consideration and trial, including electrodialysis, reverse osmosis, ion exchange, increased biological uptake, adsorption, precipitation with rare earth elements, controlled algal systems and controlled biological release.
None of these methods has attained widespread commercial significance. Under present technology all require either prohibitive capital costs, operating cost, or delicate control. Those procedures which are in commercial use are generally not widely applied, because of the problems and high costs associated with the methods as well as lack of general applicability.
Presently, the most frequently employed method of phosphorus removal is precipitation by the addition of metal oxides, such as calcium, magnesium, or sodium aluminate, or by the addition of metal salts such as iron or aluminum as chlorides or sulfates.
These chemicals have been applied at various stages of the treatment process, including the primary sedimentation stage, the secondary system prior to secondary clarification or as a tertiary stage requiring an additional solids liquid separation step.
The disadvantages inherent in all these methods are numerous and well-known.
The metal oxides are all strongly alkaline and effect a change in pH which must be accounted for or managed so as not to interfere with other functions of the process. Regulation of pH frequently requires additional equipment, controls, other chemical treatment, and additional operator attention.
The metallic salts are all strongly acid. Dealing with them involves problems comparable to those resulting from the use of metal oxides. The acidic conditions that result can impart similar limitations on the process. Addition of these chemicals in the primary or tertiary stages requires accurate proportioning of chemical dosage to influent phosphorus loading for efficient use of the chemicals. Sophisticated controls and/or continuous operator attention is required for efficient operation. In many cases effective removals of phosphorus require the additional use of flocculating chemicals such as polyelectrolytes, frequently in conjunction with carefully metered inorganic chemicals.
Application of these chemicals as a tertiary stage usually requires additional, or larger than otherwise necessary equipment, such as clarifiers, filters, or strainers, with the necessary associated equipment.
The removal of phosphorus by any of these methods produces additional sludge which must be disposed of, usually at significant cost. The precipitate formed in the use of the metal salts is voluminous and does not compact well by the usual means employed. Consequently, significantly larger volumes of sludge must be disposed of, particularly when the metal salts are applied to the primary or tertiary stages of treatment. These voluminous solids separate from the waste water with difficulty and so more costly separating equipment is required.
Frequently, when metal oxides are applied, a significant portion of the supplied cation is not utilized in the reaction and remains in solution as a contaminant, reducing the value of the treated water for subsequent re-use and creating undesirable effects in the receiving water system. When metal salts are used none of the anions are utilized, and remain in solution with similar, though perhaps more serious detrimental results. Other chemicals, which are frequently required for the efficient use of the prime chemicals, add to the detrimental effect.
In all except unusual circumstances, these chemicals are relatively costly, and their use represents a significant part of the total treatment cost.
For all these reasons phosphorus removal has not been practiced in waste treatment systems to an extent commensurate with the apparent public need or interest.